Princess Diana's funeral was held at Westminster Abbey on Sept. 6, 1997, one week after she was killed in a car crash in Paris. Elton John, who had been a friend of hers, was asked by the Royal Family to sing at the funeral.

John and lyricist Bernie Taupin rewrote "Candle in the Wind," John's 1973 homage to Marilyn Monroe, as a tribute to the Princess of Wales, and John recorded what became one of the bestsellling singles in history immediately after the funeral.

The singer and songwriter became friends with Diana in 1981 when he sang at the 21st birthday party of her brother-in-law, Prince Andrew. In the ITV documentary Diana, Our Mother: Her Life and Legacy, John said that Diana had the ability "to make people feel at ease and make them feel that everything is going to be all right. I haven't experienced many people in my life who have that ability. She could walk into a room of people and make them feel as if everything was great."

The months before her death were an emotional time for John and Diana. On July 15, their friend Gianni Versace was murdered. Diana sat next to John and comforted him during the designer's memorial service. "She kept her cool for me at Gianni's funeral and she held her composure," John told ABC's Barbara Walters. "I've got to do the same for her. When I started crying and she put her arms around me, that's exactly what she was like. She was such a caring and a very calming person. And now I've been asked to sing. Which is quite a daunting thing but I want to."

"Candle in the Wind" immediately came to the singer's mind, but everyone agreed that a song about Marilyn Monroe was inappropriate for the occasion. So John called Taupin at his Los Angeles home and asked him to write new lyrics to honor Diana. "I thought it was very important to project it from a nation's standpoint," Taupin recalled in The Songwriter's Idea Book. "I wanted to make it sound like a country singing it."

The new version, called "Candle in the Wind 1997," referred to Diana as "England's rose." Taupin wrote, "You called out to our country, and you whispered to those in pain," a reference to Diana's compassion and charitable works.

John has praised Diana's work with the AIDS charity London Lighthouse. "It was considered to be a gay disease," he said. "And for someone who was within the Royal Family – and who was a woman and who was straight – and to have someone care from the other side was an incredible gift."

Watch Elton John Perform 'Candle in the Wind'

As he prepared to sing at the funeral, John worried that he might lapse back to the original lyrics of "Candle in the Wind." "When I did the rehearsal the day before, I insisted on a teleprompter because I thought, if I sing this, and I sing 'Goodbye Norma Jean,' because I didn't know what kind of state I was going to be in on the day of the actual funeral," John told CNN's Larry King. "I just thought, I've got to get this right.

"And it was an amazing thing. I had to call on all my kind of years of professionalism, and all my experience, because I didn't want to break down and become emotional. I was doing the job of making everybody else feel emotional. And if I started to be emotional, I thought that would make it – it would denigrate the occasion."

"Me playing at the funeral was one of the most surreal things I have ever done," John recalled in a VH1 documentary years later. "What was going through my mind was, 'Don't sing a wrong note. Be stoic. Don't break down and just do it to the best you can possibly do it without showing any emotion whatsoever.' My heart was beating quite a lot, I have to say."

John has said that his performance of "Candle in the Wind 1997" at Diana's funeral would be the last time he would sing the song. "Unless I'm specifically asked by either her family or her children, I think it's time to put it to rest," he noted. "I don't want to keep evoking that memory."

The day before the service, West London's Townhouse Studios was booked for a recording session the following afternoon. Following the funeral, John and producer George Martin met at Townhouse to record "Candle in the Wind 1997." Martin wrote an arrangement for a string quartet and a woodwind player to accompany John's piano and vocals. The basic track was recorded in two takes. The second verse introduced the string section, and John overdubbed his voice on the chorus.

"Candle in the Wind 1997" was released on Sept. 23, 1997, as a double-A side with "Something About the Way You Look Tonight." The profits would go to the Diana, Princess of Wales Memorial Fund. The song was an immediate hit of historic proportions, remaining at No. 1 on the Billboard Hot 100 for 14 weeks. More than 33 million copies of the song have been sold worldwide, second only to the 50 million copies sold of Bing Crosby's "White Christmas." The song earned John the 1997 Grammy Award for Best Male Pop Vocal Performance.

"I think it was essential that that single came out as soon as possible so that people could have a remembrance of her," said John. "I wish to hell it hadn't had to come out and I wish to hell we hadn't had to go through this. I would give everything I own and go back to starting again if I could have my friends back."